Along with the development of medical theories such as the radiophysics, radiobiology, clinical oncology etc., together along with the development of medical imaging apparatus and computer technology, radiotherapy (hereinafter referred as RT) technology is continuously developed to satisfy clinic requirement better. Due to a great development from conventional RT technology to 3D conformal radiation therapy (3DCRT), RT technology becomes more precise. Therefore, both a partial recurrence rate of tumor and an occurrence rate of normal tissue complication are greatly reduced. Intensity-modulated radiation therapy (IMRT), developed on 3DCRT, is able to conform radiation to the size, shape and location of a target region more precisely, to protect OAR (Organ At Risk) around the target region more effectively.
A basic principle of IMRT is dividing the radiation field (beam field) into multiple small segment fields (beamlets) with different intensities to thereby optimize radiation delivery. In this way, the intensity of a beamlet through OAR is reduced while the intensity of a beamlet through the target region is increased. Multi-leaf collimator (MLC) is introduced to radiate a fluence map in IMRT. Especially to a target region having a recess in which OAR is positioned, IMRT technology can be performed more effectively.
The fluence map used in IMRT is possibly shaped into a U-shape or an O-shape, to leave a recess for OAR. The fluence map is discretized with zero fluence region and non-zero fluence region, as shown in FIG. 1. It is called subsectional fluence distribution.
According to conventional field dividing radiation method, in order to radiate the radiation field having subsectional fluence distribution, the radiation field is divided into two or more segment fields. Firstly, one segment field is radiated. Subsequently, jaws and MLC are moved to the next segment field when radiation is closed. Jaws comprises parallel Jaws moving along a direction parallel to a moving direction of the MLC, and perpendicular jaws moving along a direction perpendicular to the moving direction of the MLC.
FIGS. 15(a)-15(f) are schematic figures showing a processes of radiating a U-shaped fluence map with the conventional field-dividing method. Firstly, the left half of the fluence map is radiated. When the left half of the fluence map has finished to be radiated, the jaws and the MLC are moved to the right half of the fluence map. In the process of moving the jaws and MLC, the radiation is closed and the fluence map is not radiated. When the jaws and MLC have been moved to the right half of the fluence map, the right half of the fluence map begins to be radiated.
However, the field-dividing method may have some defects. In the method, a beam field may be divided into a plurality of segment fields. Radiation of the plurality of segment fields may increase total MU. For example, the total MU (Monitor Unit) may increase approximately one time to the original minimum total MU once one beam field is added. Besides, if radiation of a segment field is completed, the jaws and the MLC may move to next segment field. The moving of the jaws and the MLC may take time. The time spent on the moving may be referred as a ‘set-up time’. The total treatment time may be increased accordingly. Further, there may be penumbra at the edge of segment field. Thus, doses delivered at the edge of adjacent segment fields may be inaccurate.